1. Field of the Invention
The invention relates to semiconductor fabrication, and more particularly to a semiconductor device such as a complementary metal-oxide-semiconductor (CMOS) image sensor with a bonding pad.
2. Description of the Related Art
Typically, a CMOS image sensor is illuminated from the front (or top) side of the silicon die. Due to processing features (metallization, polysilicon, diffusions, etc.), the pixel area is partially obscured, resulting in a loss of photons reaching the sensitive area and a reduction in the area in which photons are effectively collected. This results in a reduction of the overall sensitivity of the sensor. If photons were collected from the backside of the pixel area, these obstacles could be overcome.
Backside illumination can, however, be difficult because of the thickness of the bulk silicon and the packaging technology that allows the backside to be exposed to the illumination source. The thickness of a typical silicon wafer must be thinned considerably in order to absorb the photons in the sensitive area. Prior approaches have utilized methods for bonding the die from a bonding pad in packages. The packages provide support to the thin die while providing unobstructed ports for illumination from the backside
U.S. Pat. No. 6,169,319 to Malinovich et al. disclose a method for producing a back-illuminated CMOS image sensor including a matrix of pixels (e.g., CMOS APS cells) that are fabricated on a semiconductor substrate. The semiconductor substrate is secured to a protective substrate by an adhesive such that the processed (front side) surface of the semiconductor substrate faces the protective substrate. With the protective substrate providing structural support, the exposed backside surface of the semiconductor substrate is then subjected to grinding and/or chemical etching, followed by optional chemical/mechanical processing, to thin the semiconductor substrate to a range of 10 to 15 microns. A transparent substrate (e.g., glass) is then secured to the backside surface of the semiconductor substrate, thereby sandwiching the semiconductor substrate between the transparent substrate and the protective substrate
There are, however, still some problems regarding the bonding quality between the bonding pad of CMOS image sensor and the bonding wire.